The world’s richest man has some advice for the world’s second-richest man: Work harder.

Elon Musk says Jeff Bezos needs to spend more time working at his space company Blue Origin and “less time in the hot tub” if the Amazon founder hopes to gain an edge in the billionaire space race.

“He does not seem to be willing to spend mental energy getting into the details of engineering,” Musk said in a Financial Times interview published Wednesday. “The devil’s in the details.”